1. Field of the Invention
This invention relates to an improved method of verifying the stress rolling of a surface of a metallic rim and, more specifically, one which includes taking and comparing hardness readings of the surface and a reference surface which has not been stress rolled.
2. Description of the Prior Art
It is well known that metallic rims of vehicle wheels or the like can be stress rolled to enhance the fatigue life of the rim which is being subjected to cyclical tensile stresses. Generally, this is accomplished by applying a compressive force to the rim in the portion thereof to be strengthened in a direction which is generally perpendicular to the direction of the tensile stresses produced therein. The application of the compressive force in this area will produce a compressive stress within the portion of the rim which is in the opposite direction to the eventual tensile stress. It has been found that "pre-stressing" in this manner enhances the fatigue life of such a rim which, during normal use, is expected to be repeatedly subjected to cyclical tensile forces.
However, although the stress rolling operation is most desirable in the formation of a vehicle wheel, there are problems concerning verification after the stress rolling has been accomplished. Vehicle wheels may be made of any number of materials such as aluminum, steel or other alloy materials. The initial material used to form the wheel may be purchased under broad specification requirements so that the material may have a general hardness or surface characteristics which fall within a fairly wide range of acceptable limits. Additionally, at various stages in the wheel production, the particular material may be subjected to heat treating or quenching or may be cold worked generally to improve its characteristics.
In any case, the process of stress rolling does not necessarily alter the surface condition of the metallic rim sufficiently to allow verification simply by visual inspection. It is, therefore, possible for the stress rolling operation to be omitted or inadequately performed and for a particular wheel to be otherwise properly manufactured for sale without having the desired stress rolling feature. Accordingly, there remains a need for a method of verifying that the desired stress rolling operation has been accomplished on a metallic rim at various stages in the manufacture of a wheel or prior to final release from production.